When a spinal disc becomes damaged due to trauma or disease, it may become necessary to replace the natural disc with a prosthesis. Such prostheses should preferably mimic the natural disc in shape and function, and several types of prostheses have been proposed. For example, Bao et al. disclosed a prosthetic spinal disc nucleus made of a hydrogel material that is implanted into an intradiscal space while the implant is dehydrated (U.S. Pat. No. 5,047,055). After the prosthesis is inserted, the hydrogel is hydrated and expands to a shape conforming to or approximating the natural nucleus. Bao et al. has also described a prosthetic nucleus having either a solid hydrogel core or a plurality of hydrogel beads surrounded by a membrane (U.S. Pat. No. 5,192,326). The prosthesis is implanted and hydrated to fill the intradiscal space. These devices rely on the natural annulus—fibrous tissue around the periphery of the natural disc—to constrain the expanded hydrogel. This essentially uncontrolled expansion creates a lateral force that acts directly on the annulus, which is typically already damaged. The additional force placed on the annulus by the prosthesis may impede healing and even cause further deterioration. In addition, it is difficult to accurately position dehydrated implants within the nucleus cavity.
Ray et al. disclosed one solution to the problems encountered by the prostheses of Bao et al. by proposing a hydrogel in a constraining jacket that expands on hydration (U.S. Pat. No. 6,602,291). Such a device is inserted into the intradiscal space in a first shape and is hydrated after insertion to assume a second shape that fills a volume less than the volume of the intradiscal space. This prosthesis may, however, still be difficult to implant properly. In addition, preparing the prosthesis outside of the patient may also create problems and requires the surgical team to make precise measurements of the implant site prior to inserting the prosthesis.
To provide improved prostheses, others have proposed a flow-able material that forms the prosthetic device. For example, Felt et al. disclosed an implant comprising a container that is inserted into the site of implantation and that is filled with a curable material, which is then cured in situ (U.S. Pat. No. 6,443,988). The shape of this implant may be manipulated in situ and its implantation is not hindered by a large size or awkward shape. Another flow-able prosthetic nuclear disc pulposus is disclosed by Milner et al. (U.S. Pat. No. 6,187,048). This implant comprises acrylates that are inserted into the intradiscal space and then induced to at least partially polymerize through the addition of a cross-linking agent. This prosthesis, however, is similar in composition to joint implants, which eventually decompose and may become mobile.
Another approach to the creation of a prosthesis that hardens in situ is disclosed by Ross et al. (U.S. Pat. No. 6,264,659). This implant is created by heating a thermoplastic material such as gutta percha to a temperature at which it becomes flow-able. The thermoplastic material is then injected into the intradiscal space and allowed to cool, thereby forming a prosthetic spinal disc nucleus. Implants such as these, however, utilize both polymers and/or additional curing agents that must be either mixed just prior to insertion or inserted separately. Still further, these implants may not be easily reversible.